Architectural or engineering plans are normally prepared on paper secured to a drafting table. A drafting table comprises a relatively large, flat, smooth drawing surface which can be inclined to suit the preferance of the individual draftsperson. Although drafting tables are usually inclined at some angle between vertical and horizontal when in use, directions will be referred to herein as if the tables were positioned vertically.
Preparing architectural or engineering drawings often requires that objects be located with respect to a vertical and horizontal axis and drawn with as much speed and accuracy as possible. Measuring instruments known as "scales" are used to position dots to accurately mark off distances along a horizontal or vertical axis. Once distances are marked off with dots using a scale, straight lines are drawn through the dot marks with the aid of a straight edge.
Drafting machines are devices which are attached to a drafting table to aid a draftsperson in preparing an architectural or engineering drawing. They combine the functions of a parallel ruler, protractor, scale and triangle. A conventional drafting machine includes a pair of straight edges mounted perpendicularly to one another. These straight edges lie flat against the drafting table and are rotatably mounted on a protractor head. A movable arm assembly is mounted on the table to support the protractor head. The majority of drafting machines are constructed so that the protractor head may be moved over the surface of the table without change in orientation by using one of two types of movable arm assemblies. The first type is not lockable and utilizes a parallel motion linkage comprising two sets of double bars with an intermediate elbow joint. The second type, which is lockable, utilizes a vertical arm which spans the height of the table and is slidably mounted on rails at the top and bottom of the table for horizontal movement and has a locking means. Relatively short horizontal and vertical straight edge scales are attached to a protractor head which is slidably attached for vertical movement to the vertical arm and has a locking means. By using the vertical arm to move the protractor head across the board, moving the protractor head along the vertical arm, and rotating the protractor head and positioning it in accordance with angular indicia marked thereon, the draftsperson can position the straight edges at desired angles with respect to the vertical and horizontal at most locations on the table.
When preparing drawings with a conventional drafting machine, a several-step process is required to locate and draw objects. Initially, horizontal and vertical reference lines are made with light pencil. A scale is then placed adjacent the reference line and dots are placed on the reference line to mark off coordinates corresponding to dimensions on the object being drawn. Once coordinates have been marked off on the reference lines, the straight edges of the drafting machine can be used to draw lines through these points on the drawing. These points may correspond, for example, to the center of a circle, the terminus of an arc, the intersection of two lines, or the end points of a line parallel to the reference lines. Scales may again be used to mark off points on such parallel lines.
Although the process described above allows a draftsperson to complete a drawing more quickly and accurately than without the aid of a conventional drafting machine, several undesirable aspects remain with such a procedure. Marking off points with a scale is a relatively slow process which is susceptible to inaccuracies. When marking off the coordinates on reference lines, for example, dots may not be accurately placed. When the straight edges are placed adjacent such dots in order to draw lines through the dots on the drawing, an additional error may occur. Additional inaccuracies may be introduced by improper reading of the scale. When marking fractional or decimal dimensions (e.g., 12 53/64 or 12.828 inches), it is often necessary to read closely spaced gradations, and errors may occur. Additionally, because the smallest gradations on architectural scales are marked only on one end of the scale rather than along its entire length, marking off two successive fractional dimensions can be either time-consuming or inaccurate. For example, to mark off a 1-foot, 5-inch line starting 1 foot-21/2 inches from a given point using a conventional scale, it is necessary either to add the two dimensions together (consuming extra time) or measure the second distance (i.e., 1 foot 5 inches) from the first distance, a procedure which will result in compounding any error which may occur in the first measurement.